1. Field of the Invention
Both pressure-controlled and stroke-controlled injection system are known for supplying combustion chambers of self-igniting internal combustion engines with fuel. As fuel injection systems, not only unit fuel injectors but also pump-line-nozzle units and storage injection systems are used. Storage injection systems (common rail injection systems) advantageously make it possible to adapt the injection pressure to the load and rpm of the engine. To achieve high specific outputs and to reduce emissions from the engine, the highest possible injection pressure is generally necessary.
2. Prior Art
For reasons of strength, the attainable pressure level in storage injection systems used at present is currently limited to about 1600 bar. To further increase the pressure in storage injection systems, pressure boosters are being used in common rail systems.
European Patent Disclosure EP 0 562 046 B1 discloses an actuation and valve assembly with damping for an electronically controlled injection unit. The actuation and valve assembly for a hydraulic unit has an electrically excitable electromagnet with a fixed stator and a movable armature. The armature has a first and a second surface which define a first and second hollow chamber, and the first surface of the armature points toward the stator. A valve connected to the armature is capable of carrying a hydraulic actuation fluid from a pump to the injection device. A damping fluid can be collected in or drained off from one of the hollow chambers of the electromagnet assembly in accordance with the respective chambers. By means of a region of a valve protruding into a central bore, the fluidic communication of the damping fluid can be selectively opened and closed in proportion to its viscosity.
German Patent Disclosure DE 101 23 910.6 relates to a fuel injection system which is used in an internal combustion engine. The combustion chambers of the engine are each supplied with fuel via fuel injectors which are acted upon via a high-pressure source; the fuel injection system of DE 101 23 910.6 also has a pressure booster, which includes a movable pressure booster piston that divides a chamber which can be connected to the high-pressure source from a high-pressure chamber communicating with the fuel injector. The fuel pressure in the high-pressure chamber can be varied by filling a return chamber of the pressure booster with fuel or by evacuating fuel from this return chamber.
The fuel injector includes a movable closing piston for opening and closing the injection openings that point toward the combustion chamber. The closing piston protrudes into a closing pressure chamber, enabling that chamber to be acted upon by pressure from fuel. As a result, a force urging the closing piston in the closing direction is attained. The closing pressure chamber and a further chamber are formed by a common work chamber; all the portions of the work chamber communicate permanently with one another for exchanging fuel.
With this embodiment, by triggering the pressure booster via the return chamber, it can be attained that the triggering losses in the high-pressure fuel system can be kept slight, compared to triggering via a work chamber that communicates intermittently with the high-pressure fuel source. Moreover, the high-pressure chamber is relieved only down to the pressure level of the high-pressure storage chamber, and not to the leakage pressure level. On the one hand, this improves the hydraulic efficiency of the fuel injector, and on the other, a faster depressurization down to the system pressure level can be accomplished, so that the time intervals between injection phases can be shortened.
With this embodiment, a variable hydraulic closing force which acts on the nozzle needle of the fuel injector is attainable. As a result, a variable nozzle opening pressure is achieved, which increases with the pressure prevailing in the high-pressure storage chamber, so that even at small quantities a high injection pressure is attained, and needle closure can be improved. To realize this hydraulic closing force at little engineering effort or expense, the pressure prevailing in the high-pressure storage chamber is applied directly to the back side of the nozzle needle. To enhance the efficiency, in this version the pressure booster is controlled via the return chamber, which then functions as a pressure booster control chamber. As a result, only the smaller return chamber, but not the large work chamber of the pressure booster, is relieved; in addition, the high-pressure region is relieved only down to the pressure prevailing in the high-pressure storage chamber, and not down to the leakage pressure level; as a result, the hydraulic efficiency of such an arrangement can be improved considerably. This leads to an injection system for self-igniting internal combustion engines with a high attainable injection pressure and simultaneously increased efficiency. For control, however, a 3/2-way valve is necessary, to assure a fast depressurization at the end of injection. In terms of production technology, however, a 3/2-way valve is very complicated to produce and is thus expensive. The requisite tolerances cannot be mastered at present in mass production.
In principle, it is possible for a pressure-boosted fuel injector of the embodiment known from DE 101 23 910.6 to be controlled with a 2/2-way valve in conjunction with a filling throttle. To speed up the restoration and to minimize the quantity lost via the filling throttle, a fill valve can advantageously be employed. When a fill valve is employed, however, a slow pressure drop results at the end of injection, down to the pressure level prevailing in the high-pressure storage chamber, which leads to poor emissions. A rapid pressure drop (rapid spill) is therefore absolutely necessary, if future exhaust gas limit values are to be met. Moreover, a depressurization that proceeds only slowly toward the end of an injection phase has the disadvantage that the mean injection pressure level is decreased considerably.